A cellular radio telephone system divides a geographical area up into cells where neighboring cells are generally allocated different operating frequencies so as to avoid interference. Because of the relatively low power communication transmissions with a particular cell, another cell spaced two or more cells away may typically re-use the same frequencies. The further apart cells re-using similar frequencies are located, the lower the interference level between them. The frequency re-use/cell pattern is important in determining the desired signal-to-interference ratio (C/I) in a cell.
As the total number of different frequencies required to construct a cell pattern to achieve a desired C/I ratio increases, the number of frequencies available for use within a cell decreases. For example, if a total of 420 frequencies are available and a 21-cell pattern is required before frequency re-use is permitted, the number of frequencies that can be used in each cell is 420/21=20. Consequently, one way of increasing capacity is to use a transmission technique that operates at a reduced C/I.
Current cellular telephone systems prefer digitized voice transmission, as compared to the transmission of analog voice waveforms, because digitized transmission tolerates more interference. Thus, digitized voice transmission allows for a smaller frequency re-use pattern with a consequent increase in system capacity. When digital transmission techniques are used, error correction coding is often employed to increase interference tolerance. Unfortunately, error correction coding effectively widens the transmitted signal frequency bandwidth, reducing the number of available frequency channels. Extra interference tolerance permitting increased re-use of frequencies must be balanced with a reduction in the number of frequencies available.
The relationship between system capacity versus the amount of error coding is not monotonic and includes several maxima and minima as the amount of error coding increases. At one extreme, the amount of error coding is so great that interference levels equal to or in excess of the power level of the desired signal can be tolerated. In that situation, overlap between signals is permissible, and the system is known as a Code Division Multiple Access (CDMA) system.
In CDMA systems with many overlapping, interfering signals, a factor of two increase in system capacity may be achieved by temporarily turning off subscriber transmitters during the moments of silence during a two-party conversation. It has been well documented that 50% or more of the time during a call connection between two subscribers is actually silence. Consequently, the number of conversations may be doubled before interference becomes problematic. A Discontinuous Transmission (DTX) takes advantage of this feature and is employed in conventional cellular access systems, such as the Time Division Multiple Access (TDMA), Pan-European Digital Cellular System known as GSM. DTX effectively reduces the prevailing interference of all the signals with respect to each signal.
Another technique for reducing interference between signals in neighboring cells on the same frequency is to configure the transmission power distribution of a cell over all of the mobiles within a cell according to each mobile's distance from its respective cell edge. The power transmitted downlink from a base station in the center of the cell to a mobile on the cell edge should be the greatest. In other mobile locations further from the cell edge and closer to the base station, the power level should follow a fourth power radius law based on the distance or radius of the mobile from the cell center where typically the base station is located. In the uplink direction from mobile to base station, the mobile's transmission power should also be set according to a fourth power radius law, in order to equalize the signal strengths received at the base station and to prevent those mobiles closest to the base station from using unnecessary power levels that would cause substantial interference.
Unfortunately, there is no direct technique for either the base station or the mobile to determine the distance between themselves. Consequently, the radius necessary to construct a fourth power law is unknown. This problem is overcome in conventional systems using a technique known as Dynamic Power Control in which a command is transmitted from the base station to the mobile to reduce its power if the signal strength received by the base station from that mobile is unnecessarily high. Similarly, the mobile sends a message to the base station including a measurement of the signal strength received from the base station. The base station uses that measurement to regulate its transmitted power to that mobile. The Dynamic Power Control technique has the disadvantage that it is slow to react because of the cumbersome, bi-directional messages needed between the base station and the mobile. The bi-directional signalling also reduces the capacity or quality of the traffic channel.
It would be desirable to have a cellular power control system that has increased system capacity in terms of frequency reuse but that minimizes the effects of any increased interference. Moreover, it would be desirable to achieve these goals by regulating efficiently and accurately the power transmitted from the base station to a mobile and the power transmitted from each mobile to its base station without the need for bidirectional power control messages between the base station and the mobiles.